Mask



FIG. 1 is a perspective view of a mask according to an embodiment of our design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof; and,

FIG. 5 is a left side view thereof.

The broken lines show environment or structure of the mask and form no part of the claimed design. 

The ornamental design for a mask, as shown and described. 